1. Field of the Invention
The invention relates to a steel tie rod end that constitutes part of a vehicle steering system and a method of manufacturing the steel tie rod end.
2. Description of the Related Art
Tie rods are components that constitute part of a vehicle steering system. The tie rods couple the steering system (for example, a steering gear of a rack and pinion steering system) to front wheel side components (for example, knuckle arms).
Rotational motion of a steering wheel is transmitted to the steering system via a steering shaft. Then, the rotational motion is converted into linear motion by the steering system, and the linear motion is transmitted to front wheels via the tie rods and the knuckle arms. Each tie rod typically includes a tie rod body and a tie rod end. The tie rod body is assembled to the steering system via a ball joint. One end of the tie rod end is assembled to the tie rod body, and the other end of the tie rod end is assembled to the knuckle arm via a second ball joint.
A typical tie rod end is made of a steel material, such as a carbon steel and a low-alloy steel, and has a shaft portion and two fitting portions. The two fitting portions are formed at both ends of the shaft portion. As described above, one of the fitting portions is assembled to the tie rod body, and the other one of the fitting portions is assembled to the knuckle arm.
Incidentally, in order to improve steering feel, it is generally considered a good approach that the rigidity of each tie rod end is decreased to increase the flexibility of each tie rod end to thereby smoothly transmit steering operation to the front wheels. Note that the rigidity indicates a necessary load with which the tie rod end is elastically deformed by a predetermined length in the axial direction (longitudinal direction). A tie rod end having a low rigidity deforms by a large amount with a small load.
Various techniques have been suggested in order to decrease the rigidity of a tie rod end (or a tie rod) (for example, see Japanese Utility Model Application Publication No. 7-37862 (JP-U-7-37862)).
JP-U-7-37862 describes a method in which the cross-sectional area of the shaft portion is reduced. Because the tie rod ends are components mounted on a vehicle, it is desirable that the cross-sectional area of the shaft portion of each tie rod end is reduced in terms of space saving and light weight. However, when the cross-sectional area of the shaft portion is just simply reduced, the strength of each tie rod end decreases. This may possibly make it difficult to ensure necessary strength. A tie rod end having a poor strength may possibly buckle or break.
In addition, when a high strength material is used as the material of the tie rod end, it is presumable that a decrease in strength of the tie rod end may be suppressed although the rigidity of the tie rod end is decreased with a reduction in the cross-sectional area of the shaft portion. However, generally, forming a tie rod end (particularly, forming fitting portions) requires machining, such as cutting. Therefore, if the high strength material is used as the material of the tie rod end, machinability deteriorates. This problematically increases cost for manufacturing the tie rod end.
Furthermore, the tie rod ends are components that receive a relatively large impact because the tie rod ends are connected to drive wheels. As described above, the tie rod ends are components that are assembled to the steering system. Thus, if any of the tie rod ends breaks because of an impact in the event of a collision, or the like, steering function is impaired. Thus, a tie rod is required to not only have the above described low rigidity and high strength but also have a high impact characteristic. Particularly, because the toughness of a steel material decreases at a low temperature, there is a need for a tie rod end that also exhibits a high impact characteristic at a low temperature.